Generally speaking, under concentrated-light operation condition, III-V multi-junction solar cell can absorb solar energy in a wider wavelength range and as a result its photoelectric conversion efficiency, that is about 43% in laboratory test, is higher than the usual flat-plate solar cell. Hence, the III-V multi-junction solar cells are especially suitable to be applied in large ground-mounted solar power systems for providing electricity in residential sector. Operationally, the electricity output of a solar cell is influenced by the intensity of the light that shines on it while the electricity output of the solar cell can be fed to and used by an external device through its metal electrodes. In a condition when a solar cell is operating especially under concentrated-light operation condition, there will be an electric current of several amperes or more flowing through the components inside the solar cell, and consequently, if there is any components inside the solar cell that is designed with poor heat dissipating ability in view of thermal resistance, the temperature of such component as well as the whole solar cell will raise after light shines on the solar cell, causing the conversion efficiency of the solar cell to deteriorate, i.e. the current-voltage characteristics of the solar cell are adversely affected.
Conventionally, for improving the thermal conducting ability of a GaAs-based solar cell, the solar cell that is formed on a raw GaAs substrate is first being attached to an adhesive layer of a metal substrate that is formed with high heat-dissipating ability by wafer bonding, and then a chemical solution, such as a mixing solution of ammonia, hydrogen peroxide and water, is used for etching the raw GaAs substrate so as to prepare the same for having a layer of metallic electrode grids to be formed therein, as shown in FIG. 1. Thereby, a structure of a solar cell attaching to a metal substrate with high heat-dissipating ability is achieved.
As the raw GaAs substrate will be etching away by the chemical solution in the aforesaid conventional solar cell manufacturing process, thus such manufacturing process is disadvantageous in that: first, the raw substrate can not be recycled and used repetitively; and second, the manufacturing cost is increased comparing with those solar cells without the metal substrate with high heat-dissipating ability, owing to the acquisition costs of the raw substrate and the metal substrate with high heat-dissipating ability, and the waste management cost for treating the arsenic-containing waste liquid resulting from the manufacturing process.
Conventionally, in order to overcome the aforesaid shortcomings, a prior-art technique is provided, in which the raw substrate is separated from the solar cell and thus removed by the selectively etching of a sacrificial layer using a chemical solution. However, since such etching of the sacrificial layer can only be performed starting from the lateral of a wafer used for forming solar cells, the lateral etching rate of the sacrificial layer can be very low due to the restriction of capillary action and the limitation relating to a minimum contact area. Therefore, it can take a very long period of time just for performing a substrate lift-off process upon a large-size wafer. Thus, such prior-art technique for separating the raw substrate might not be feasible for industrial mass production.